How can I UV Unwrap a Minecraft box model?
You have to mark seams to create the unwrap like this. Select the edges you want to place seams on, then press Ctrl+E-->Mark Seam. For more tips about how to place seams right see: How to Place the Right Seams for UV Unwrapping. After placing seams to one side of your mesh apply the Mirror modifier. Alternatively you may start from appying it and then mark seams.


Next select the whole mesh (A) and press U-->Unwrap. As you can see you get the result similar to the one you provided. If you want to make it identical you have to experiment with seams placement.


Here's the .blend with my seams placement example: